774 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Using the experiments of Gouy and Chaperon as a basis, Walker has offered 

 a similar explanation of basic contact-shells in intrusive masses.* Substituting 

 for the Gouy and Chaperon's principle the principle of liquation (or limited 

 miscibility within a certain range of magmatic temperature), the present writer 

 finds Walker's explanation applicable to the vast majority of basic contact-shells 

 in the larger injected and subjacent bodies. 



Each of these shells may, then, be regarded as that part of the magma 

 in which marginal cooling checked gravitative differentiation, while the more 

 slowly cooled magma occupying the great central part of the magma chamber 

 underwent a more thorough separation of the salic and femic constituents. 

 Examples have been noted in the Osoyoos batholith (page 441), the Similkameen 

 batholith (page 457), and the Castle Peak stock (page 494). Contact ' basification ' 

 bas often been observed in large vertical dikes, where the combined influence 

 of contact chilling and gravitative differentiation may again be the explanation. 

 The strong chemical contrast between wall phase and middle phase and the 

 structure of many dikes suggest, however, that the differentiation has been 

 facilitated by special concentration of gases in the interior part of each dike. 

 In such cases the volatile matter doubtless increased the fluidity and hastened 

 the magmatic splitting. 



The accompanying diagrams (Figure 42) will make the conception clearer. 



Chemical Contrast of Plutonic and Corresponding Effusive Type. — To gravi- 

 tative differentiation we may ascribe the steady chemical differences between 

 plutonic rocks and the corresponding effusives. The latter must come from the 

 highest levels of magma columns. They are, accordingly, somewhat richer in 

 silica and alkalies, and poorer in iron oxides, lime, and magnesia than their 

 respective deep-seated equivalents. This important fact is illustrated in Table 

 LV. The chemical contrast between the respective pairs of rocks can hardly 

 be explained as the result of mere diffusion on the Soret principle or any other. 

 Diffusion undoubtedly controls the growth of crystals but only rarely can it be 

 credited with the segregation of special magmas on the scale of igneous-rock 

 bodies. 



* T. L. Walker, Amer. Jour. Science, Vol. 6, 1898, p. 410. 



